1. Technical Field
The present invention relates to techniques for preventing a discharge lamp from being continuously applied with excessive power more than necessary due to failure in a load when the discharge lamp is turned on from a cold state.
2. Background Art
When a discharge lamp is used for car illumination, light flux must be rapidly increased after the discharge lamp is turned on, so that a transient power control is conducted to supply larger power immediately after the discharge lamp is lit than the power applied for a normal lighting situation, and further, to reduce the applied power over time.
For example, for a metal halide lamp with rated power of 35 W, when lighting is started from a state in which its light emitting tube is cold (so-called “cold start”), the power is controlled such that approximately 60 to 80 W of power is transiently applied to the lamp. Then, the applied power is gradually reduced based on a control value calculated in accordance with the state of the lamp (mainly, a lamp voltage) and an elapsed time from the power-on time to eventually converge the power to the rated value.
Power loss of a lighting circuit increases as larger power is outputted. When large power is applied to a lamp, as with the cold start, a large loss occurs, which increases the amount of generated heat.
In a related art lamp lighting process, even if increased power is temporarily applied in a transient period, the transient period lasts for several seconds, so that the lighting circuit has sufficiently durable specifications with respect to heat-resisting designs of the lighting circuit. Even so, measures should be taken for a possible failure in a load.
For example, in the event of a leak or submergence of a bulb, troubles caused by manufacturing-related faults and aging deterioration (an excessively small amount of mercury, a reduction in inner pressure of an arc tube, and the like), a failure in a lamp, and the like, or when an accident brings about an equivalence to a parallel circuit of a lamp and a low resistor in a lamp connector, a lamp voltage detected by a detector circuit included in a lighting circuit indefinitely remains low, possibly resulting in continuous application of excessive power to the lamp.
In the related art (for example, see Laid-open Japanese Utility Model Registration Application No. 7-8997), a discharge lamp is monitored for power supplied thereto, such that the power supply to the discharge lamp is shut off when the discharge lamp is supplied with power that exceeds the power that should be supplied after the discharge lamp has been lit. Alternatively, when there is a function of determining whether a lamp voltage falls within a normal range in a static light condition determined from information on a detected lamp voltage, the lamp voltage equal to or lower than a predefined reference value is regarded as a failure in lighting, and the power supplied to the discharge lamp can be shut off to protect the circuit.
However, the related art techniques have problems of insufficient measures for protecting circuits from heat generated by a failed load, an increased cost therefor, and the like.
For example, when a high-frequency switching scheme is employed to reduce the size of a lighting apparatus having a fly-back type DC-DC converter circuit, continued application of excessive power, in the event of a failure, can directly lead to thermal runaway, thermal destruction, and the like, due to a reduced thermal capacitance of the overall apparatus.